I agree with the Supreme Court that the federal government should recognize state-approved gay marriages. But I am not persuaded by the court's rationale for concluding that such a policy is constitutionally required. In fact, I'm not sure what its rationale is.

For supporters of gay marriage (most of us, according to recent polls), the fuzziness of the court's reasoning can be seen as an advantage, since it opens the door to requiring state as well as federal recognition of same-sex unions. But if and when the court takes that step, the lack of a clear and convincing constitutional argument will invite discord and disrespect.

The strongest argument against Section 3 of the Defense of Marriage Act (DOMA), which defines marriage for all federal purposes as a union between a man and a woman, is that it improperly interferes with state regulation of domestic relations. When it passed DOMA, Congress explicitly sought to uphold a traditional view of marriage and implicitly sought to deter states from recognizing gay unions.

In his majority opinion, Justice Anthony Kennedy initially seems to be making such a federalist argument, emphasizing that domestic law has always been the domain of the states. But after building that argument for seven pages, he suddenly veers away, saying, "It is unnecessary to decide whether this federal intrusion on state power is a violation of the Constitution because it disrupts the federal balance."

If he had stuck with federalism, Kennedy might have alienated the four left-leaning justices who joined his opinion: Stephen Breyer, Ruth Bader Ginsburg, Sonia Sotomayor and Elena Kagan. The five of them still could have concurred in the result while disagreeing on the reasoning, but perhaps Kennedy wanted to avoid the messiness of multiple opinions.

Or perhaps, as Justice Antonin Scalia suggests in his dissent, Kennedy wanted to set the stage for a future ruling overturning state bans on gay marriage. A federalist argument would not have done that. To the contrary, it would have implied that the issue of gay marriage should be decided by the states without interference by the federal government one way or the other.

The obvious alternative was the argument that DOMA, by dictating discrimination against gay couples, violated the principle of equal protection. By and large, that is the tack Kennedy seems to prefer, although he also throws in vague references to "the liberty protected by the Due Process Clause."

Because laws routinely treat different categories of people differently, the court has developed a three-tier system of analysis that allows most distinctions if there is a "rational basis" for them -- a highly deferential test that requires only that a law be rationally related to a legitimate government purpose. At the other extreme is discrimination based on "suspect classes," such as race or national origin, which receives "strict scrutiny," meaning it must be "narrowly tailored" to achieve a "compelling" government interest.

In between is "intermediate scrutiny" (aka "heightened scrutiny"), which is used in equal protection cases involving gender and requires that a legal distinction be "substantially related" to an "important governmental objective." The plaintiff in this case, joined by the Justice Department, argued that discrimination based on sexual orientation should be subject to heightened scrutiny.

Although the appropriate level of scrutiny was an issue throughout the history of this case, Kennedy never addressed it. But since he says the motivation for DOMA amounted to nothing more than "a bare congressional desire to harm a politically unpopular group," he seems to think the law fails the rational-basis test.

One problem with this description, as Scalia and Justice Samuel Alito note in their dissents, is that it portrays DOMA's supporters -- and, by implication, supporters of state bans on gay marriage -- as bigots with no rational basis for their position. In a country where at least two-fifths of the population still opposes gay marriage, that may not be a winning message.